Short-Term Borrowings-Schedule (Details) - USD ($) $ in Millions	3 Months Ended												12 Months Ended
	Dec. 31, 2019	Sep. 30, 2019	Jun. 30, 2019	Mar. 31, 2019	Dec. 31, 2018	Sep. 30, 2018	Jun. 30, 2018	Mar. 31, 2018	Dec. 31, 2017	Sep. 30, 2017	Jun. 30, 2017	Mar. 31, 2017	Dec. 31, 2019	Dec. 31, 2018	Dec. 31, 2017
Short-term Debt [Line Items]
Short-term borrowings	$ 3,659				$ 4,180				$ 4,650				$ 3,659	$ 4,180	$ 4,650
Securities sold under repurchase agreements [Member]
Short-term Debt [Line Items]
Short-term borrowings	$ 1,072	$ 5,568	$ 3,553	$ 3,714	$ 1,466	$ 2,471	$ 2,743	$ 2,792	$ 3,365	$ 6,494	$ 7,558	$ 4,069	$ 1,072	$ 1,466	$ 3,365
Weighted average interest rate, point in time (percent)	1.75%				4.16%				2.10%				1.75%	4.16%	2.10%
Average during year	$ 1,483	$ 8,432	$ 3,433	$ 3,142	$ 3,045	$ 3,550	$ 5,008	$ 5,182	$ 5,457	$ 12,566	$ 9,272	$ 5,050	$ 4,130	$ 4,189	$ 8,100
Weighted average interest rate, during the period (percent)													3.28%	2.86%	1.28%
Maximum month-end balance													$ 8,569	$ 5,829	$ 11,351
Commercial paper [Member]
Short-term Debt [Line Items]
Short-term borrowings	$ 2,476				$ 2,532				$ 1,154				$ 2,476	$ 2,532	$ 1,154
Weighted average interest rate, point in time (percent)	2.21%				2.70%				1.56%				2.21%	2.70%	1.56%
Other [Member]
Short-term Debt [Line Items]
Short-term borrowings	$ 111				$ 182				$ 131				$ 111	$ 182	$ 131